I had this issue and it was overboost problem caused by sticking vanes on the turbo, mainly due rust and soot.
So, when driving it hard (4th gear climbing a hill) the engine cutted-off when reaching 3000-3500rpm, and literally die for a few seconds.
So most sure you'll have to disassembly the exhaust collectors and turbo, then break the turbo and check it.
The compressor wheel ,the clean side that goes to the filter/intake pipe should be shiny, use some degreaser to clean it.
The exhaust part, may be full of soot and rust, so use some metallic brush to clean the turbine and the vnt vanes.
And the body of the exhaust is pure iron, I used a metallic brush with the drill to clean it as most as possible.
Now, months after I did this, I'll have to open it again because the old vnt vanes were worn but I didn't had cash to fix at that time. It is causing slow turbo spool.